A gas-insulated switchgear (hereafter, referred to as a “GIS”) has devices, such as a breaker, disconnecting switch, earthing switch and the like. The GIS often uses a 3-position switch wherein an earthing switch and a disconnecting switch are united in a sealed tank.
FIG. 5 shows the outline of the 3-position switch that has been commonly used. The 3-position switch is constructed such that in an sealed tank 101 having a tank length of L1, there are provided three-phase main circuit conductors 102 extending in the direction shown in the drawing and a main circuit conductor 103 disposed so that the extended axes thereof intersects with the main circuit conductor 102. Also, the 3-position switch has a disconnecting switch including a disconnecting switch-side fixed contact 110a provided on the main circuit conductor 102 side and a disconnecting switch-side moving contact 107a that linearly reciprocates in the disconnecting switch-side conductor 103a. Furthermore, on the other main circuit conductor 103 and the sealed tank 101 side, there is provided an earthing switch including an earthing switch-side fixed contact 110b and an earthing switch-side moving contact 107b that linearly reciprocates in the earthing switch-side conductor 103b. 
Moreover, an operating shaft 104 is rotatably disposed between the disconnecting switch and the earthing switch. A one-hole lever 105 that is connected to the operating shaft 104 and allows an arc motion as shown by the dashed-dotted line is connected to each end of rectilinear links 106a and 106b; the other end of the rectilinear link 106a is connected to the disconnecting switch-side moving contact 107a; and the other end of the rectilinear link 106b is connected to the earthing switch-side moving contact 107b. This structure enables the disconnecting switch-side moving contact 107a and the earthing switch-side moving contact 107b to linearly reciprocate as the operating shaft 104 rotates.
In the 3-position switch shown in FIG. 5, an angle formed by the central axis line of the disconnecting switch-side conductor 103a and the central axis line of the grounding-side conductor 103b intersecting with each other (hereafter, referred to as an “open angle”) is a blunt angle much larger than 90 degrees. Therefore, there was a problem in that the tank length L1 of the sealed tank 101 becoming large, increasing the size of the entire GIS.
Accordingly, as shown in FIG. 6, it is considered possible to make the tank length L2 of the sealed tank 201 approximately 80% of the length of the tank shown in FIG. 5 by making an open angle between the disconnecting switch-side conductor 203a and the grounding-side conductor 203b nearly a right angle. However, in this structure, at the initial motion of the disconnecting switch-side moving contact 207a and the earthing switch-side moving contact 207b, the frictional force between the disconnecting switch-side moving contact 207a and the cylindrical sliding surface of the disconnecting switch-side conductor 203a as well as the frictional force between the earthing switch-side moving contact 207b and the cylindrical sliding surface of the grounding-side conductor 203b becomes significantly great. Therefore, there was a problem in that an operating device having a large drive output to operate the operating shaft 204 is necessary, causing the entire GIS to become large. Furthermore, there was also a problem in that due to the friction between the moving contacts 207a, 207b and the cylindrical sliding surfaces of the conductors 203a, 203b, respectively, foreign objects that affect insulation characteristics could easily be produced. Herein, in FIG. 6, components that correspond to the same portions in FIG. 5 are numbered in the 200s, and their description will be omitted.
A structure in which an open angle between the disconnecting switch-side conductor and the grounding-side conductor is nearly a right angle has been realized, for example, in a 3-position switch described in the publication of examined applications No. Showa 54(1979)-29701 (patent literature 1). Patent literature 1 discloses a 3-position switch including a cam having a nearly V-shaped cam groove provided between central conductors, and a disconnecting switch-side moving contact and an earthing switch-side moving contact that move in the cam groove by means of rollers.
This structure makes it possible to reduce the length of the tank by making the open angle between the disconnecting switch-side conductor and the grounding-side conductor nearly a right angle. However, there was a problem in that sliding powder was generated as the result of the rollers sliding on the cam groove. Furthermore, another problem was that the structure was too complicated and it took time to produce.
Next, a load force generated by driving a 3-position switch shown in FIG. 6 will be described with reference to the enlarged views of the switch portion and the vector diagrams shown in FIG. 7 through FIG. 9. FIG. 7(B) is a vector diagram in which an initial motion torque is given counterclockwise to the operating shaft 204 in the grounding state where the earthing switch-side moving contact 207b has been entered into the earthing switch-side fixed contact 210b as shown in FIG. 7(A).
Herein, a force component and a reaction force on the disconnecting switch portion 209a side will be discussed. In FIG. 7(B), when an initial drive torque is given, drive force F0 is generated at the position of the rotation pin 211c of the single-hole lever 205. Next, force component F11 of the drive force F0 is generated, and force component F12 indicated by F11 Cos θ2 and force component F13 indicated by F11 Sin θ2 are further generated. Herein, θ2 is an angle formed by the center line of the disconnecting switch-side moving contact 207a and the line connecting rotation pins 211a and 211c. 
It is preferable that the force component F12 be large because it becomes an effective propulsion force in the direction of the axis of the moving contact 207a. However, the problem is the force component F13 that is generated in the direction perpendicular to the axis of the moving contact 207a. Due to the force component F13, the moving contact 207a is subject to reaction force F14 and reaction force F15 from the sliding surface of the disconnecting switch-side conductor 203a. 
On the other hand, on the earthing switch portion 209b side, since the earthing switch-side moving contact 207b behaves in an opposite manner from the disconnecting switch portion 209a, force component F21 is generated from drive force F0; then, force component F22 indicated by F21Cosθ1 and force component F23 indicated by F21Sin1 are generated. Herein, θ1 is an angle formed by the center line of the earthing switch-side moving contact 207b and the line connecting rotation pins 211b and 211c. 
It is preferable that the force component F22 be large because it becomes an effective propulsion force in the direction of the axis of the earthing switch-side moving contact 207b. However, the problem is the force component F23 that is generated in the direction perpendicular to the axis of the earthing switch-side moving contact 207b as previously stated. Due to the force component F23, the earthing switch-side moving contact 207b is subject to reaction force F24 and reaction force F25 from the sliding surface of the earthing switch-side conductor 203b. 
Next, the disconnecting state shown in FIGS. 8(A) and 8(B) will be described. When comparing angles θ1 and θ2 in this state with angles θ1 and θ2 in FIG. 7(B), angles shown in FIG. 8(B) are smaller. The angles θ1 and θ2 being small means that the sliding frictional force is small. This is because the sliding frictional force is a function of angles θ1 and θ2.
Next, the closed state shown in FIGS. 9(A) and 9(B) will be described. When comparing angles θ1 and θ2 in this state shown in FIG. 9(B) with angles θ1 and θ2 in FIG. 7(B), angle θ1 in FIG. 9(B) is the same as angle θ2 in FIG. 7(B), and angle θ2 in FIG. 9(B) is the same as angle θ1 in FIG. 7(B). This is because this link mechanism has an axisymmetric structure with respect to the bisector of the angle formed by the disconnecting switch-side conductor 203a and the grounding-side conductor 203b. Therefore, the reaction force that the disconnecting switch-side moving contact 207a receives from the sliding surface of the disconnecting switch-side conductor 203a is equivalent to the reaction force shown in FIG. 7(B).
The sliding frictional force is the product of reaction forces F14, F15, and F24, F25 that moving contacts 207a, 207b receive, respectively, and the contact friction coefficient of the cylindrical inner surface of the disconnecting switch-side conductor 203a and the cylindrical inner surface of the earthing switch-side conductor 203b, respectively. Since angles θ1, θ2 shown in FIG. 7 through FIG. 9 change according to the rotation position of the single-hole lever 205, the reaction forces F14, F15, F24, F25 also change according to the rotation position of the single-hole lever 205. The above study indicates that the angles θ1, θ2 are largest at the initial motion of each moving contact and at the completion of the operation; accordingly, the sliding frictional force also becomes largest at the initial motion of each moving contact and at the completion of the operation.
Hereinafter, based on FIGS. 10(A), (B), and (C), the relationship between the operation of each moving contact 207a, 207b and a load torque will be described. FIG. 10 shows the change of load torque Tb due to a sliding frictional force when a constant drive torque Ta is provided by an operating device. Let the friction coefficient between moving contacts 207a, 207b and the cylindrical sliding surface of the disconnecting switch-side conductor 203a and the cylindrical sliding surface of the earthing switch-side conductor 203b, respectively, be 1.2.
This load torque Tb curve shows the change of load torque when the 3-position switch starts operating from the grounding state. Load torque Tb in FIG. 10(A) shows only a load torque on the disconnecting switch portion 209a side. When drive torque Ta of the operating device is 100%, load torque Tb at the initial motion is 93.5%. As the disconnecting switch-side moving contact 207a moves in the closed-circuit direction, which is the direction of the disconnecting switch-side fixed contact 210a, load torque Tb rapidly decreases; and when angle θ2 shown in FIG. 3 through FIG. 5 is at the zero point, force component F13 becomes zero. Although load torque Tb tends to increase after angle θ2 passes the zero point, the torque is obviously much smaller than the load torque at the initial motion.
FIG. 10(B) shows only a load torque on the earthing switch portion 209b side when the same operation shown in FIG. 10(A) is conducted. The load torque curve in FIG. 10(B) is completely opposite from that in FIG. 10(A). FIG. 10(B) indicates that when drive torque Ta of the operating device is 100%, the load torque only on the earthing switch portion 209b side is 93.5% immediately before the operation is completed.
When the, operating shaft 204 rotates, both the load torque of the disconnecting switch portion 209a and the load torque of the earthing switch portion 209b are simultaneously applied to the operating shaft 204. Load torque Tb plotted in FIG. 10(C) is the sum of the load torques in FIG. 10(A) and FIG. 10(B) that have been arithmetically calculated. At the initial motion of the 3-position switch, that is, when the stroke of each moving contact 207, 207b is 0% (immediately after operation has started from the grounding state), load torque Tb is 99.7% with respect to drive torque Ta of 100%. As the stroke of the disconnecting switch-side moving contact 207a increases, load torque Tb decreases. However, during 40% to 60% of the stroke, load torque Tb stops decreasing and starts to increase; and when the stroke is 100% (immediately before the closed state), load torque Tb reaches 99.7%.
As the above study indicates, an extremely large load torque occurs in the conventional 3-position switch shown in FIG. 6 at the initial motion and at the completion of the operation. Therefore, the conventional 3-position switch with a single-hole lever shown in FIG. 6 must use an operating device having a large operation force, which resulted in a problem that the size of the operating device increases.
An objective of a 3-position switch according to the present invention is to prevent the generation of foreign objects by maximally suppressing a sliding frictional force between the moving contact and the hollow conductor while adopting a simple mechanism to rectilinearly move both the moving contact of the disconnection portion and the moving contact of the earthing switch portion in an interlocking manner, thereby reducing the size of the entire apparatus including an operating device.